GE Engineer Who Worked on #Fukushima I Nuke Plant Reactors: "Japan May Have Lost Race"

From UK's Guardian (3/29/2011, emphasis added, complete article at the link):

The radioactive core in a reactor at the crippled Fukushima nuclear power plant appears to have melted through the bottom of its containment vessel and on to a concrete floor below, experts say, raising fears of a major release of radiation at the site.

Richard Lahey, who has worked on the plant at Fukushima, told the Guardian officials seemed to have "lost the race" to save the reactor, but added that there was no danger of a Chernobyl-style catastrophe.

.... At Fukushima, workers have been pumping water into three reactors in a desperate bid to keep the fuel rods from melting down. But Lahey, who was head of safety research for boiling-water reactors at General Electric when the company installed the units at the plant, said his analysis of radiation levels suggested these attempts had failed at reactor two.

He said at least part of the molten core, which includes melted fuel rods and zirconium alloy cladding, seemed to have sunk through the steel "lower head" of the pressure vessel and on to the concrete floor below.

"The indications we have, from the reactor to radiation readings and the materials they are seeing, suggest that the core has melted through the bottom of the pressure vessel in unit two, and at least some of it is down on the floor of the drywell," Lahey said. "I hope I am wrong, but that is certainly what the evidence is pointing towards."

The major concern when molten fuel breaches a containment vessel is that it will react with the concrete floor of the drywell, releasing radioactive gases into the surrounding area. At Fukushima, the drywell has been flooded with seawater, which will cool any molten fuel that escapes from the reactor and reduce the amount of radioactive gas released.

Lahey said: "It won't come out as one big glob; it'll come out like lava, and that is good because it's easier to cool." [I hope he's right about that.]

The drywell is surrounded by a secondary steel-and-concrete structure designed to keep radioactive material from escaping into the environment. But an earlier hydrogen explosion at the reactor may have damaged this.

"The reason we are concerned is that they are detecting water outside the containment area that is highly radioactive and it can only have come from the reactor core," Lahey added. "It's not going to be anything like Chernobyl, where it went up with a big fire and steam explosion, but it's not going to be good news for the environment."

The radiation level at a pool of water in the turbine room of reactor two was measured recently at 1,000 millisieverts per hour. At that level, workers could remain in the area for just 15 minutes, under current exposure guidelines.

A less serious core meltdown happened at the Three Mile Island nuclear plant in Pennsylvania in 1979. During that incident, engineers managed to cool the molten fuel before it penetrated the steel pressure vessel. The task is a race against time, because as the fuel melts it forms a blob that becomes increasingly difficult to cool.

In the light of the Fukushima crisis, Lahey said all countries with nuclear power stations should have "Swat teams" of nuclear reactor safety experts on standby to give swift advice to the authorities in times of emergency, with international groups co-ordinated by the International Atomic Energy Authority.

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